Various systems associated with internal combustion engines rely on a supply of hydraulic fluid, an example of which includes engine oil. For the sake of brevity, the specific example of engine oil is used throughout this disclosure, though it is understood that other fluids are possible.
With efforts to reduce parasitic losses, many engines (including diesel engines) have smaller oil pumps and very low oil pressure available for supplying various systems with oil, including engine brake systems. As known in the art, various engine brake systems or other systems capable of varying the opening and closing times of engine valves (i.e., so-called Variable Valve Actuation (VVA) systems) often rely on one or more hydraulic lost motion components. More specifically, these lost motion components are used to vary the length of a valve train path between a valve actuation motion source and an engine valve. “Lost motion” is a term applied to a class of technical solutions for modifying the valve motion dictated by the otherwise fixed profile of a valve actuation motion source using a variable length mechanical, hydraulic, or other linkage means. A lost motion system may comprise a variable length device included in the valve train linkage between the valve actuation motion source and the engine valve. The fixed valve lift profile of the valve actuation motion source may provide the maximum motion (i.e., longest time between opening and closing as well as the largest lift for any particular valve event) needed for a range of engine operating conditions. When expanded fully, the variable length device within the valve train may transmit all of the valve actuation motion to the valve, and when contracted fully, transmit none or a reduced amount of the valve actuation motion to the valve. By selectively decreasing the length of the lost motion system, part or all of the valve actuation motion can be effectively subtracted or “lost.”
Hydraulic-based lost motion systems may provide a variable length device through use of a hydraulically extendable and retractable assembly. For example, in one embodiment, a hydraulic-based lost motion system may utilize a hydraulic circuit, including a master piston and a slave piston, that is selectively charged with hydraulic fluid to actuate an engine valve. When the hydraulic circuit is charged with hydraulic fluid, a hydraulic lock between the master and slave pistons may be created. Given the relatively incompressible nature of the hydraulic fluid, valve actuation motions applied to the master piston are conveyed to the slave piston and, subsequently, the engine valve. On the other hand, the master and slave circuit may be depleted of hydraulic fluid when it is desired to lose the valve actuation motion input to the master piston. Under rapidly changing operating conditions, it often becomes necessary to quickly charge or deplete the hydraulic fluid used to operate such hydraulic-based lost motion systems.
However, as noted above, the availability of only relatively low pressure hydraulic fluid systems often makes the timely charging of hydraulic lost motion systems difficult. It is known to incorporate larger hydraulic supply lines through external components (relative to the engine itself) to provide improved pressure. However some engines have comparatively low pressure even in the main hydraulic fluid supply and such external components cannot increase oil pressure above the main hydraulic fluid supply.